This invention relates to the preparation of low molecular weight liquid polymers derived from at least one acrylic monomer.
The preparation of polymeric materials from acrylic monomers, such as methacrylate esters, is well known in the prior art. Typically, such polymerization processes are conducted at atmospheric pressure, and temperatures below about 120.degree. C, and usually comprise the use of free-radical initiating reagents. Such polymerizations may be conducted in bulk, in solution, or in aqueous suspensions or emulsions. The recovered polymeric products are suitable for use as molding plastics, etc. However, because of their relatively high softening temperatures and/or high viscosities, the utility of such polymeric products in coating applications is largely limited to use in multi-component coating vehicles, in which solvents, emulsifiers, water, or other materials are also present.
In accordance with this invention, there is prepared a low molecular weight, low viscosity, liquid polymer which is especially useful as a non-drying, single-component fugitive coating vehicle.
More particularly, in accordance with this invention there is prepared a low molecular weight liquid polymeric material by the polymerizing of at least one selected acrylic monomer under a polymerization pressure in excess of atmospheric pressure and at a temperature not exceeding the ceiling temperature of the polymeric material at the polymerization pressure.
Typically in the practice of this invention, an acrylic monomer, such as an alkyl methacrylate, is subjected to thermal polymerization in an enclosed vessel with the polymerization temperature near, but slightly below, the `ceiling temperature` of the polymer under the reaction conditions, such that the competition between propagation and depropagation processes leads to the formation of a polymer of low molecular weight.
For any given polymer-monomer system, there exists a `ceiling temperature` at which the rate of polymerization becomes equal to the rate of depolymerization; see, for example, R. W. Lenz, `Organic Chemistry of Synthetic High Polymers`, Interscience Publishers, 1967, pages 325-329. At or near the ceiling temperature, the competition between polymerization and depolymerization processes leads to a lowering of the polymerization rate and of the molecular weight of the resulting polymer.
In conventional polymerization processes it is rarely possible to control molecular weight by the use of the ceiling temperature phenomenon, since the ceiling temperature is higher than the boiling point of the corresponding monomer(s). However, the evaporation of a volatile monomer can be avoided by carrying out the polymerization in an enclosed vessel, and/or by employing elevated pressures. The use of elevated pressure has the further effect of altering the ceiling temperature; that is, ceiling temperatures tend to increase at higher pressures.
Typically the process of this invention is conducted in an enclosed vessel such that the polymerization pressure can be derived autogenously by heating the monomer (or monomers) and/or the selective addition of an inert gas such as nitrogen.
A great advantage of this invention is that it may be conducted in the absence of a polymerization catalyst. However, it is anticipated that a catalyst may be utilized. As used herein, the term catalyst is intended to include reaction or polymerization initiators, promoters, etc. Typical catalysts include dialkyl peroxides, diacyl peroxides, peroxyesters, organic hydroperoxides, organic azo compounds, and metallo-organic reagents.
The process of this invention is most suitably conducted at polymerization pressures substantially in excess of atmospheric, usually about 2 to about 10 atmospheres, and at high elevated polymerication temperatures close to but not exceeding the ceiling temperature of the to-be-prepared polymer, usually about 200.degree. C to about 300.degree. C.
As used herein, the term "acrylic monomer" signifies an ester or other derivative of acrylic acid or of methacrylic acid.
Examples of suitable acrylic monomers include the following: